In conventional methods and apparatus for digitally encoding repetitive analog waveforms, the analog waveforms are sampled at a plurality of sampling points, and each analog sample is encoded as a multi-bit digital word. The analog sample must be held for the duration of the complete multi-bit encoding operation.
Generally, a sample and hold circuit is provided to hold the analog sample during the multi-bit encoding operation. Sample and hold circuits suitable for sampling high frequency waveforms typically include high speed components such as Schottky bridges, step recovery diodes and pulse transformers which must be electromagnetically isolated from one another to prevent undesired cross-coupling. The requirement for electromagnetic isolation and the construction of some of the components (e.g. the pulse transformers) are not compatible with monolithic integration of the sample and hold circuits, so the cost of such sample and hold circuits can be quite high, particularly where many repetitive waveforms must be sampled and encoded in parallel.
The required holding time for the analog sample can be reduced by deriving all bits of the multi-bit digital word simultaneously as in "flash" analog to digital conversion. Unfortunately, flash analog to digital converters employ a large number of comparators connected in parallel to derive all bits of the multi-bit digital word simultaneously, and this parallel connection of comparators contributes a large parasitic capacitance which limits the maximum operating speed. Moreover, flash analog to digital converters are now rather expensive for applications in which several repetitive waveforms must be encoded in parallel.
For example, in a tester described in detail below, electromagnetic emissions are sensed simultaneously by over one thousand probes, and the resulting analog waveforms are digitally encoded in parallel. If relatively expensive sample and hold circuits or flash analog to digital converters are required for each encoding channel, the overall cost impact could be very significant.